On Axial Diffusion in Laminar-Flow Reactors

Although unimportant in conventional laminar-flow reactors, axial diffusion can be the dominant mechanism for mixing in micrometer-scale devices. The low Reynolds numbers characteristic of these devices allow numerical solution of the convective diffusion equation, coupled when necessary with the equations of motion. The method of false transients provides a simple way to obtain such numerical solutions. For the case of reaction within a confined zone, there is an internal optimum with respect to diffusion. Unlike the lumped-parameter axial dispersion model, the conversion and residence time distribution depend on the boundary conditions. The physically realizable open-open boundary conditions give higher conversions and more uniform residence times. Micrometer-scale devices will have such high levels of diffusion that mixing between separately fed, compatible components will become trivially easy. Reactor performance will approximate that of a CSTR with attendant low yields and poor selectivities.